The study also implies that most Americans are consuming a perfectly
healthy amount of salt, the main source of sodium. But those who are
salt-sensitive, about 20 to 25 percent of the population, still need to
restrict salt intake.

Consuming fewer than 2,500 milligrams of sodium daily is actually associated with higher blood pressure, according to the Framingham Offspring Study report, given today.
The American Heart Association recommends consuming no more than 2,300
milligrams of sodium daily, equal to a teaspoon of ordinary iodized
table salt.

High blood pressure is a known risk factor for heart disease
and stroke. Hence, lowering salt intake is supposed to lower blood
pressure and thus reduce the risk of cardiovascular disease and stroke.
But the study found that supposition to be unfounded.

Moreover, the lowest blood pressure was recorded by those who
consumed 4,000 milligrams or more a day — amounts considered dangerously
high by medical authorities such as the American Heart Association.

Those taking from 2,500 milligrams to 4,000 milligrams a day had very
slightly higher blood pressure, but significantly below the low-sodium
group. The average American consumes 3,400 milligrams of sodium a day.

Higher levels of calcium, potassium and magnesium were also associated
with lower blood pressure. The lowest readings came from people who
consumed an average of 3,717 milligrams of sodium and 3,211 milligrams
of potassium a day.

The study is an offshoot of the groundbreaking Framingham Heart Study. Both are projects of the National Heart Lung and Blood Institute and Boston University.

The new report was delivered in Chicago during the Experimental Biology
meeting by Lynn L. Moore, an associate professor of medicine at Boston
University School of Medicine.

The report directly contradicts advice from the American Heart
Association, which recommends consuming less than 1,500 milligrams of
sodium a day to reduce blood pressure and risk of heart disease.

This story was originally published at 8:30 a.m. Tuesday. It was most recently updated at 2:30 p.m.

The NEJM study examined 412 participants for 30 days. They were randomly
assigned to eat either a control diet or the Dietary Approaches to Stop
Hypertension (DASH) diet, which is rich in vegetables, fruits, and
low-fat dairy products, in persons with and in those without
hypertension.

The Framingham Offspring Study based its findings on a
population of more than 2,600 men and women, whom it followed for 16
years. That means it can capture the long-term results of salt
consumption, which the New England Journal of Medicine study couldn't do because of its short duration.

Response

Cheryl Anderson, a member of the American Heart Association's Nutrition
Committee, said the study appeared to have some weaknesses in data
collection, according to its abstract. Anderson didn't attend the
presentation, because she was delivering her own at the same time.

The best standard for such studies of sodium intake is a 24-hour
multiple urine collections protocol, which the study didn't have, said
Anderson, an associate professor in the Department of Family and
Preventive Medicine at UC San Diego.

The study used dietary records, which aren't as accurate as direct measurement, Anderson said.

And it's also possible that some of those in the study may have changed
their sodium intake because they developed high blood pressure, which
would confound the correlation, she said.

"When I put it in the broader context of the general literature around
dietary sodium assessment and blood pressure, particularly from what we
know about clinical trials, (the study) didn't bother me in any way,"
Anderson said.

"Until I see the whole paper, this is not going in any way to disrupt my
current thinking around that fact that high sodium intakes are likely
to increase your blood pressure."

Anderson pointed to studies that examined the relationship between
sodium intake and the outcome of cardiovascular disease events. These
include a study published in the journal BMJ in 2007, led by Nancy Cook of Brigham and Women's Hospital, Harvard Medical School in Boston.

That study enrolled 744 participants with prehypertension, randomized to
either a low-sodium diet or control. It included an original
measurement of sodium and subsequent followups.

"The crude rate of cardiovascular disease was somewhat lower among those
assigned to the sodium reduction intervention in stratified analysis)
than corresponding controls," the study stated. "After adjustment for
baseline characteristics, particularly the imbalance in age, there were
significant differences between groups."

That study began with urinary measurements of sodium excretion, but in
the final long-term followup questionnaire, the study used self-reported
information on sodium intake.

Anderson said ideally, such studies outcomes would have followed
the 24-hour urine collection protocol, but that has proven too
difficult. People would need to be monitored for decades to detect the
long-term effects of dietary sodium, and that would be prohibitively
expensive.

"Are you going to give me money for 25 years to do that?"
Anderson asked. "That's why these studies haven't been done. It's
because they're logistically quite a challenge, from a funding
perspective almost impossible."

Extrapolation invalidated

Moore, who delivered the report at the Experimental Biology meeting,
said the putative link between higher sodium consumption, higher blood
pressure, and cardiovascular disease and stroke wasn't well understood.
More recent evidence suggests the logic behind that link is flawed.

Early studies found that among those with high blood pressure, restricting sodium lowered blood pressure.

"A low-sodium diet has been popularized for a few decades, and I think
it originally had to do with the observation that among people with high
blood pressure, if you restrict their sodium intake, their blood
pressure actually goes down."

"I think that led to the inference that restricting your sodium intake
in general (among the public) would actually reduce your risk of
developing high blood pressure," Moore said.

Further extrapolating, since higher blood pressure is a risk factor for
cardivascular diseases, then lowering sodium would lower blood pressure
and hence lower cardiovascular disease rates.

Meanwhile, evidence undermined the extrapolation, Moore said.
The picture turned out to be much more complicated. People on the
lowest-sodium diets actually appeared to have higher cardiovascular risk
and risk of dying than those taking more sodium.

"There's been consequently a lot of controversy over the last
five or six years over this topic, but there is a growing body of
evidence suggesting that the relationship between sodium and
cardiovascular disease is actually J-shaped."

Moore was referring to a pattern in which an extremely low level of the
substance being measured produces a higher risk, which falls to a
plateau and then rises again at the higher extreme.

This is the pattern found in an August 2014 study in the New England Journal of Medicine,
which found a greatly elevated risk of death or major heart event at
the dietary sodium level recommended by the American Heart Association.

The study also found that the average intake for Americans placed them near the bottom of the risk curve.

Anderson, the American Heart Association scientist, said that study was
also flawed because it didn't use a 24-hour urine collection protocol.

Biologically determined?

Moore said greater attention needs to be given to a hypothesis that
people generally consume the amount of sodium they need. In other words,
they are biologically driven to keep their consumption within a certain
range.

The J-shaped curve implies that tampering with this drive could cause unforeseen health problems.

"There's evidence that salt restriction has a lot of effects on
other systems other than blood pressure," Moore said. "You end up with higher levels of renin, rather than lower levels," referring to an enzyme that helps raise blood pressure.

"Other studies have shown that cholesterol goes up, triglyceride
levels go up,. So there are a number of effects on known risk factors
for heart disease that are independent of blood pressure, that seem to
be activated in a setting of salt restriction," she said.

The 2010 Dietary Guidelines for Americans recommended that sodium intake
be lowered to 2,300 milligrams per day for the general population. ,
The report is a joint project of the U.S. Departments of Health and
Human Services and of Agriculture.

"Overall, the committee found that both the quantity and quality of
relevant studies to be less than optimal," the IOM report delicately
stated.

But the carefully worded report also concluded that the bulk of
the evidence indicates a correlation between higher levels of sodium
intake and cardiovascular disease.

It also said there was insufficient evidence to conclude that
lowering sodium intake below 2,300 milligrams per day either increases
or decreases the risk of cardiovascular disease or death in the general
population.